Process for the production of formaldehyde

ABSTRACT

A process is described for the production of formaldehyde, comprising (a) subjecting methanol to oxidation with air in a formaldehyde production unit thereby producing a formaldehyde-containing stream; (b) separating said formaldehyde-containing stream into a formaldehyde product stream and a formaldehyde vent gas stream; wherein the vent gas stream, optionally after treatment in a vent gas treatment unit, is passed to one or more stages of: (i) synthesis gas generation, (ii) carbon dioxide removal, (iii) methanol synthesis or (iv) urea synthesis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/552,072, filed Aug. 18, 2017, which is a national stage applicationof International Patent Application No. PCT/GB2015/054082, filed Dec.18, 2015, which claims the benefit of priority of Great Britain PatentApplication No. 1502894.7, filed Feb. 20, 2015, the disclosures of whichare hereby incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to a process for the production offormaldehyde. More particularly, it relates to a process for theproduction of formaldehyde which may be integrated into a flow-sheet forthe production of formaldehyde-containing products, such as, forexample, formaldehyde-stabilised urea in a process including theco-production of methanol and ammonia.

BACKGROUND

Urea finds widespread use as a fertiliser and in industrial chemicalmanufacture. It is conventionally made by reacting ammonia with carbondioxide to form a solid product which is often shaped by prilling orgranulating. Formaldehyde or a urea-formaldehyde concentrate (UFC) areoften used to stabilise the urea before or during the shaping process.UFC is also used as a feedstock for the manufacture of urea-formaldehyderesins.

Formaldehyde is manufactured by the oxidation or dehydrogenation ofmethanol. There is scope for providing an integrated process in whichthe production of formaldehyde is integrated with a flowsheet involvingthe production of one or more formaldehyde-containing precursors orproducts, leading to more efficient use of energy and/or of feedstocks.

However, the demand for formaldehyde to stabilise the urea from a singleproduction facility is small and beyond the economic feasibility for adedicated formaldehyde production facility. Due to the small scale ofthe requirements, the formaldehyde is normally produced at a separatededicated formaldehyde production facility and transported to theammonia/urea production facility where it is stored. An integratedurea-formaldehyde process with a dedicated formaldehyde production unitbased on a methanol-ammonia co-production process which improves theammonia productivity and does not reduce urea production therefore hasthe potential to provide efficiency savings compared with a conventionalprocess

SUMMARY

Accordingly the invention provides a process for the production offormaldehyde, comprising (a) subjecting methanol to oxidation with airin a formaldehyde production unit thereby producing aformaldehyde-containing stream; (b) separating saidformaldehyde-containing stream into a formaldehyde product stream and aformaldehyde vent gas stream; wherein the vent gas stream, optionallyafter treatment in a vent gas treatment unit, is passed to one or morestages of: (i) synthesis gas generation, (ii) carbon dioxide removal,(iii) methanol synthesis or (iv) urea synthesis.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings in which;

FIG. 1 is a schematic representation of a process according to a firstaspect of the present invention; and

FIG. 2 is a schematic representation of a process according to a secondaspect of the present invention.

It will be understood by those skilled in the art that the drawings arediagrammatic and that further items of equipment such as reflux drums,pumps, vacuum pumps, temperature sensors, pressure sensors, pressurerelief valves, control valves, flow controllers, level controllers,holding tanks, storage tanks, and the like may be required in acommercial plant. The provision of such ancillary items of equipmentforms no part of the present invention and is in accordance withconventional chemical engineering practice.

DETAILED DESCRIPTION

In one embodiment, the invention provides a process for the productionof formaldehyde-stabilised urea, comprising (a) subjecting methanol tooxidation with air in a formaldehyde production unit thereby producing aformaldehyde-containing stream; (b) separating saidformaldehyde-containing stream into a formaldehyde product stream and aformaldehyde vent gas stream; (c) synthesising urea in a urea productionunit; and (d) stabilising the urea by mixing the urea and a stabiliserprepared using formaldehyde recovered from said formaldehyde productstream, wherein the formaldehyde vent gas stream, optionally aftertreatment in a vent gas treatment unit, is passed to one or more stagesof: (i) synthesis gas generation, (ii) carbon dioxide removal, (iii)methanol synthesis or (iv) urea synthesis employed in the production ofurea or formaldehyde.

There have been numerous designs for ammonia and methanol co-productionover the last 50 years or so, but they have generally focussed ongenerating large quantities of both materials as saleable products.Examples of such processes are described, for example in U.S. Pat. Nos.6,106,793, 6,333,014, 7,521,483, 8,247,463, 8,303,923, andWO2013/102589. None of these processes include a dedicated formaldehydeproduction unit with vent gas recycle as claimed. As the presentinvention utilises recycle of the vent gas from the formaldehydeproduction unit, substantial benefits in the reduction of capital andoperating costs are possible, in particular where the vent gas stream isrecycled directly to the process.

In a conventional formaldehyde production process, the formaldehydeproduction unit comprises a formaldehyde reactor, in which methanol isoxidised to produce formaldehyde, and a separation unit in which theformaldehyde product is separated from the formaldehyde vent gas. Theformaldehyde vent gas separated from the formaldehyde product stream maycontain nitrogen, oxygen, carbon monoxide, water and organics includingmethanol, formaldehyde and dimethyl ether. In a typical formaldehydeproduction process, the formaldehyde vent gas stream may be recirculatedto the formaldehyde reactor and a portion vented to the atmosphere,usually after treatment in a vent gas treatment unit such as anemissions control system (ECS). The ECS may comprise non-selectiveoxidation of the organic compounds in the formaldehyde vent gas streamin order to remove them from the gas prior to atmospheric venting. It isusually necessary to treat at least some of the formaldehyde vent gas inthis way because the recirculated gas contains methanol and theadmixture with air needs to avoid the risk of explosion; therefore it isnot possible to recirculate all of the vent gas to the formaldehydereactor. The ECS represents a major capital cost in the building of aformaldehyde plant and requires operation and maintenance, therebyadding to the cost of the formaldehyde production unit. Advantageously,in the process of the invention, the formaldehyde vent gas need not betreated in an ECS.

The process may comprise a synthesis gas generation step in a synthesisgas generation unit, and conversion of the synthesis gas to methanol foruse in step (a) of the process. The synthesis gas may also be used toproduce ammonia, in particular for the production of urea using carbondioxide recovered from the synthesis gas.

The synthesis gas preferably comprises hydrogen, nitrogen, carbonmonoxide, carbon dioxide and steam. The synthesis gas may be generatedby any suitable means. The synthesis gas generation may be based onsteam reforming of a hydrocarbon such as natural gas, naphtha or arefinery off-gas; or by the gasification of a carbonaceous feedstock,such as coal or biomass. Preferably the synthesis gas generation stagecomprises steam reforming a hydrocarbon. This may be achieved by primaryreforming a hydrocarbon with steam in externally-heated catalyst-filledtubes in a fired- or gas-heated steam reformer and, where the methanecontent of the primary reformed gas is high, secondary reforming theprimary-reformed gas mixture in a secondary reformer, by subjecting itto partial combustion with an oxygen-containing gas and then passing thepartially combusted gas mixture through a bed of steam reformingcatalyst. The oxygen-containing gas may be air, oxygen oroxygen-enriched air. The formaldehyde vent gas stream from theformaldehyde production unit may be used in the synthesis gas generationunit as a feedstock or alternatively as a fuel for heating a reformerunit.

Whereas secondary reforming with air or oxygen-enriched air usefullyprovides the nitrogen in the reaction stream, the synthesis gas may beproduced by primary steam reforming or autothermally reforming ahydrocarbon feed using oxygen alone and providing nitrogen from anothersource, such as an air separation unit (ASU).

The primary reforming catalyst typically comprises nickel at levels inthe range 5-30% wt, supported on shaped refractory oxides, such as alphaalumina, magnesium aluminate or calcium aluminate. If desired, catalystswith different nickel contents may be used in different parts of thetubes, for example catalysts with nickel contents in the range 5-15% wtor 30-85% wt may be used advantageously at inlet or exit portions of thetubes. Alternatively, structured catalysts, wherein a nickel or preciousmetal catalyst is provided as a coated layer on a formed metal orceramic structure may be used, or the catalysts may be provided in aplurality of containers disposed within the tubes. Steam reformingreactions take place in the tubes over the steam reforming catalyst attemperatures above 350° C. and typically the process fluid exiting thetubes is at a temperature in the range 650-950° C. The heat exchangemedium flowing around the outside of the tubes may have a temperature inthe range 900-1300° C. The pressure may be in the range 10-80 bar abs.In a secondary reformer, the primary-reformed gas is partially combustedin a burner apparatus mounted usually near the top of the reformer. Thepartially combusted reformed gas is then passed adiabatically through abed of steam reforming catalyst disposed below the burner apparatus, tobring the gas composition towards equilibrium. Heat for the endothermicsteam reforming reaction is supplied by the hot, partially combustedreformed gas. As the partially combusted reformed gas contacts the steamreforming catalyst it is cooled by the endothermic steam reformingreaction to temperatures in the range 900-1100° C. The bed of steamreforming catalyst in the secondary reformer typically comprises nickelat levels in the range 5-30% wt, supported on shaped refractory oxides,but layered beds may be used wherein the uppermost catalyst layercomprises a precious metal, such as platinum or rhodium, on a zirconiasupport. Such steam reforming apparatus and catalysts are commerciallyavailable.

Alternatively, the steam reforming maybe achieved by passing a mixtureof the hydrocarbon and steam through an adiabatic pre-reformercontaining a bed of steam reforming catalyst and then passing thepre-reformed gas mixture to an autothermal reformer which operates inthe same way as the secondary reformer to produce a gas streamcontaining hydrogen, carbon oxides and steam. In adiabaticpre-reforming, a mixture of hydrocarbon and steam, typically at a steamto carbon ratio in the range 1-4, is passed at an inlet temperature inthe range 300-620° C. to a fixed bed of pelleted nickel-containingpre-reforming catalyst. Such catalysts typically comprise ≥40% wt nickel(expressed as NiO) and may be prepared by co-precipitation of anickel-containing material with alumina and promoter compounds such assilica and magnesia. Again, the pressure may be in the range 10-80 barabs.

Alternatively, the reaction stream may be formed by gasification ofcoal, biomass or other carbonaceous material with air using gasificationapparatus. In such processes the coal, biomass or other carbonaceousmaterial is heated to high temperatures in the absence of a catalyst toform a crude synthesis gas often containing sulphur contaminants such ashydrogen sulphide, which have to be removed. Gasification ofcarbonaceous feedstock to produce a synthesis gas may be achieved usingknown fixed bed, fluidised-bed or entrained-flow gasifiers attemperatures in the range 900-1700° C. and pressures up to 90 bar abs.The crude synthesis gas streams require additional treatments known inthe art to remove unwanted sulphur and other contaminants.

In a preferred process, the synthesis gas generation stage comprisesprimary reforming a hydrocarbon, particularly natural gas, in a firedsteam reformer to produce a gas stream comprising hydrogen, carbonmonoxide, carbon dioxide and steam, and secondary reforming stage inwhich the primary reformed gas is further reformed in a secondaryreformer using air or oxygen-enriched air to provide a synthesis gasstream comprising hydrogen, carbon oxides and nitrogen.

The process may include a stage of removing carbon dioxide from thesynthesis gas. Before recovery of the carbon dioxide, the crudesynthesis gas is preferably subjected to one or more stages of water-gasshift to produce a shifted synthesis gas with the desired gascomposition. In a water gas shift stage, a portion of the carbonmonoxide in the stream is converted to carbon dioxide. Any suitablecatalytic shift conversion reactor and catalyst may be used. Ifinsufficient steam is present, steam may be added to the gas streambefore it is subjected to the water-gas shift conversion. The reactionmay be depicted as follows;H₂O+CO

H₂+CO₂

The vent gas from the formaldehyde production unit typically containssteam and CO which may be used in the feed to the water-gas shiftreaction. The reaction may be carried out in one or more stages. The, oreach, stage may be the same or different and may be selected from a hightemperature shift process, a low temperature shift process, a mediumtemperature shift process and an isothermal shift process.

High temperature shift catalysts may be promoted iron catalysts such aschromia- or alumina-promoted magnetite catalysts. Other high temperatureshift catalysts may be used, for example iron/copper/zinc oxide/aluminacatalysts, manganese/zinc oxide catalysts or zinc oxide/aluminacatalysts. Medium, low temperature and isothermal shift catalyststypically comprise copper, and useful catalysts may comprise varyingamounts of copper, zinc oxide and alumina. Alternatively, where sulphurcompounds are present in the gas mixture, such as synthesis gas streamsobtained by gasification, so-called sour shift catalysts, such as thosecomprising sulphides of molybdenum and cobalt, are preferred. Suchwater-gas shift apparatus and catalysts are commercially available.

For high temperature shift catalysts, the temperature in the shiftconverter may be in the range 300-360° C., for medium temperature shiftcatalysts the temperature may be in the range 190-300° C. and forlow-temperature shift catalysts the temperature may be 185-270° C. Forsour shift catalysts the temperature may be in the range 200-370° C. Theflow-rate of synthesis gas containing steam may be such that the gashourly space velocity (GHSV) through the bed of water-gas shift catalystin the reactor may be ≥6000 hour⁻¹. The pressure may be in the range10-80 bar abs.

In a preferred embodiment, the water-gas shift stage comprises a hightemperature shift stage or a medium temperature shift stage or anisothermal shift stage with or without a low temperature shift stage.

Steam present in the shifted synthesis gas mixture may be condensed bycooling the shifted gas to below the dew point using one or more heatexchangers fed, for example, with cooling water. The condensate may berecovered in a gas liquid separator and may be fed to steam generatorsthat produce steam for the synthesis gas or water-gas shift stages.

A carbon dioxide removal unit may be used to recover carbon dioxide fromthe synthesis gas. If present, it is located downstream of the synthesisgas generation, preferably downstream of a water-gas shift stage, andupstream of a methanol synthesis unit. Any suitable carbon dioxideremoval unit may be used. Suitable removal units may function byreactive absorption, such as those known as aMDEA™ or Benfield™ unitsthat are based on using regenerable amine or potassium carbonate washes,or by physical absorption, based on using methanol, glycol or anotherliquid at low temperature, such as Rectisol™, Selexol™ units. Carbondioxide removal may also be performed by means of pressure-swingadsorption (PSA) using suitable solid adsorbent materials. The carbondioxide removal unit, if operated using physical absorption at lowtemperature, is able to simultaneously remove residual steam in theshifted synthesis gas by condensation. Such carbon dioxide removalapparatus and materials are commercially available. Some or all of thecarbon dioxide formed in the synthesis gas may be removed to produce agas stream comprising mainly hydrogen and nitrogen with low levels ofcarbon monoxide. The carbon dioxide removed by the carbon dioxideremoval unit may be captured, treated to remove contaminants such ashydrogen, and stored or used for reaction downstream with the ammoniaproduced to form urea.

It is desirable to remove water from the carbon dioxide-depletedsynthesis gas. Water removal, or drying, is desirable to protect thedownstream methanol synthesis catalyst, improve the kinetics of themethanol synthesis reaction and to minimise water in the crude methanolproduct. Water removal may also improve the performance and reliabilityof the first stage of compression. Water removal may be accomplished bycooling the water-containing gas below the dew point using one or morestages of heat exchange and passing the resulting stream through a gasliquid separator. Further stages of drying, e.g. with a desiccant may beperformed if desired.

Methanol may be synthesised from the carbon dioxide-depleted synthesisgas. Any methanol production technology may be used. Methanol issynthesised in a methanol synthesis unit, which may comprise a methanolconverter containing a methanol synthesis catalyst. The process can beon a once-through or a recycle basis in which unreacted product gas,after optional condensate removal, is mixed with make-up gas comprisinghydrogen and carbon oxides in the desired ratio and returned to themethanol reactor. The methanol synthesis, because it is exothermic, mayinvolve cooling by indirect heat exchange surfaces in contact with thereacting gas, or by subdividing the catalyst bed and cooling the gasbetween the beds by injection of cooler gas or by indirect heatexchange. The methanol may be recovered by condensation. The methanolsynthesis also produces water, which may also be fed to the formaldehydeproduction unit. The synthesis gas composition preferably hasPH₂>2PCO+3PCO₂ such that there is excess hydrogen to react with theoxides of carbon. The stoichiometry number, R, as defined byR=([H₂]-[CO₂])/([CO]+[CO₂]), of the synthesis gas fed to the methanolsynthesis catalyst, is preferably ≥3, more preferably ≥4, mostpreferably ≥5.

An unreacted gas stream may be recovered from the methanol synthesisunit as a methanol synthesis off-gas. A methanol synthesis off-gas maytherefore comprises nitrogen, hydrogen and residual carbon monoxide.

A purge gas stream may be removed to prevent the undesirable build-up ofinert/unreactive gases. If desired methanol may also be synthesised fromthis purge gas, or hydrogen recovered from it, for example to adjust thestoichiometry of the resulting gas or to generate power.

Any methanol synthesis catalyst may be used, but preferably it is basedon a promoted or un-promoted copper/zinc oxide/alumina composition, forexample those having a copper content in the range 50-70% wt. Promotersinclude oxides of Mg, Cr, Mn, V, Ti, Zr, Ta, Mo, W, Si and rare earths.In the catalyst, the zinc oxide content may be in the range 20-90% wt,and the one or more oxidic promoter compounds, if present, may bepresent in an amount in the range 0.01-10% wt. Magnesium compounds arepreferred promoters and the catalyst preferably contains magnesium in anamount 1-5% wt, expressed as MgO. The synthesis gas may be passed overthe catalyst at a temperature in the range 200-320° C., and at apressure in the range 20-250 bar abs, preferably 20-120 bar abs, morepreferably 30-120, bar abs and a space velocity in the range 500-20000h⁻¹. When the aim of the process is not to maximise methanol production,the inlet temperature of the methanol synthesis stage may be lower, e.g.200-270° C. thus extending the catalyst lifetime by reducing sinteringof the active copper sites.

A single stage of methanol synthesis may be sufficient. Nevertheless, ifdesired, the methanol synthesis may be part of a multiple synthesisprocess where the product gas, with or without condensate removal, isfed to one or more further methanol synthesis reactors, which maycontain the same or different methanol synthesis catalyst. Such methanolproduction apparatus and catalysts are commercially available.

Crude methanol recovered from a methanol synthesis unit isconventionally purified by multiple stages of distillation. When therecovered methanol is oxidised to make formaldehyde, it is possible tosimplify the methanol purification process. Therefore crude methanolrecovered from the methanol synthesis stage may be used directly in theformaldehyde production unit without further purification. If desiredhowever, the crude methanol may be subjected one or more purificationstages, including a single de-gassing stage, in a methanol purificationunit prior to feeding it to the oxidation reactor. The de-gassing stageor any distillation stages may be provided by distillation columnsheated using heat recovered from the oxidation reactor or elsewhere inthe process. In particular, the degassing stage may be heated usingsteam generated by the oxidation stage. This simplification of thepurification offers significant savings in capital and operating costsfor the process.

Methanol is oxidised to formaldehyde in step (a). Any formaldehydeproduction technology may be used. The formaldehyde is synthesised informaldehyde production unit, which may comprise an oxidation reactorcontaining an oxidation catalyst. The oxidation catalyst may be providedas a fixed bed or preferably, within externally cooled tubes disposedwithin the reactor. Air, or air enriched in oxygen and methanol may bepassed to the reactor containing an oxidation catalyst in which themethanol is oxidised.

Production of formaldehyde from methanol and oxygen may be performedeither in a silver- or a metal oxide catalysed process operated atmethanol-rich and methanol-lean conditions, respectively. Hence theoxidation catalyst may be selected from either a silver catalyst or ametal oxide oxidation catalyst, preferably an oxidation catalystcomprising a mixture of iron and molybdenum oxides. Vanadium oxidecatalysts may also be used. In the metal oxide process the principalreaction is the oxidation of the methanol to formaldehyde;2CH₃OH+O₂→2CH₂O+2H₂O

Over silver catalysts, in addition to the above oxidation reaction,methanol is also dehydrogenated in the principal reaction for this typeof catalyst;CH₃OH→CH₂O+H₂

In the metal oxide process, formaldehyde is produced in multi-tubereactors. Typically, a reactor comprises 10-30,000 preferably 10-20,000tubes filled up with ring-shaped or other shaped catalysts and cooled byoil or molten salts as heat transfer fluid. Since the reaction is highlyexothermic (ΔH=−156 kJ/mol), isothermal conditions are difficult toobtain and consequently a hotspot may be formed within the reactionzone. In order to limit the hot spot temperature, at the first part ofthe reactor the catalyst can be diluted with inert rings. The catalystused in the oxide process is preferably a mixture of iron molybdateFe₂(MoO₄)₃ and molybdenum trioxide MoO₃ with a molybdenum: iron atomicratio between 2 and 3. In most aspects the catalytic performance issatisfactory; the plant yield is high (88-93 or 94%) and neithermolybdenum nor iron are toxic, which is favourable considering bothenvironmental and human health aspects.

Air is preferably used at levels to maintain the oxygen content at theinlet of the reactor below the explosive limit. The feed gas maytherefore comprise ≤6.5% by volume methanol for a once-through reactoror about 8-11% by volume, preferably 8-9% by volume methanol where thereis recirculation. The oxidation reactor may be operated adiabatically orisothermally, where the heat of reaction can be used to generate steam.The inlet to the oxidation reactor is typically in the range 80-270° C.,preferably 150-270° C., with iron-based catalytic processes operating upto 400° C. and silver-based processes up to 650° C. The operatingpressures are typically 1.1-5 bar abs, preferably 1.3-5 bar abs.

A single passage through the oxidation reactor can result in high yieldsof formaldehyde, or if desired it is possible to recycle unreactedgases, which comprise mainly nitrogen, from the vent gas separator tothe reactor inlet to maintain a low oxygen concentration. Thus anoxygen-lean gas may be recovered from e.g. an absorber column andrecycled to the feed stream to the reactor, e.g. to a feed line wherethe recycle gas is mixed with fresh air. A low oxygen concentrationpermits a higher methanol concentration at the reactor inlet. Forexample, if the oxygen content is about 11% vol, then the methanolconcentration may be about 6.5% vol, but if the oxygen content is 13%vol or higher such concentrations are not suitable because the feed gasmixture is within the explosive limits.

Depending on the scale required in the present process, the stage may beoperated with or without recycle of oxidised gas to the inlet of theoxidation reactor. Operation without recycle may be beneficial insmaller processes because this removes the need for a recycle compressorand hence offers further savings.

The formaldehyde—containing stream produced in the formaldehyde reactoris separated into a formaldehyde product stream and a vent gas. Theseparation unit may comprise absorption to extract the formaldehydeproduct from the oxidised gas mixture into either water to produceaqueous formaldehyde solution, or a urea solution to produce aurea-formaldehyde concentrate (UFC). The absorption may be performedusing an absorption tower, which may contain a selection of packing,trays and other features to promote the absorption, and cooling watermay be used to provide the product at a temperature in the range 20-100°C. The absorption stage typically runs at a slightly lower pressure thanthe reactor.

Urea formaldehyde concentrate that may be produced typically comprises amixture of about 60% wt formaldehyde, about 25% wt urea and the balanceabout 15% wt water. Such a product may be termed “UFC85”. Other UFCproducts may also be produced. Other formaldehyde products may also beproduced. Excess formaldehyde products may be sold. The products madefrom the formaldehyde may be used to stabilise urea.

The formaldehyde production unit generates a vent gas. The vent gas mayoptionally be treated in a vent gas treatment unit such as an emissioncontrol system (ECS). An ECS may comprise a catalytic combustor thatreacts any carbon monoxide, methanol, formaldehyde and dimethyl ether inthe vent gas with oxygen. The gas emitted from an ECS, i.e. an ECSeffluent, typically comprises carbon dioxide, steam and nitrogen andtherefore may be recycled, preferably after suitable compression, to theprocess. Thus the ECS effluent may be passed to a carbon dioxide-removalstage of a synthesis gas generation unit where steam and carbon dioxidemay be recovered, to provide additional nitrogen in the synthesis gas.Alternatively the ECS effluent may be provided to a methanol synthesisunit where the carbon dioxide may be reacted with hydrogen in thesynthesis gas to produce additional methanol. Alternatively, the ECSeffluent may be fed to the urea production unit to provide carbondioxide for additional urea production. The ECS effluent may be providedto one or more of these alternatives.

In another embodiment, the vent gas treatment unit comprises agas-liquid separator that separates the nitrogen-rich off-gas fromliquid methanol, which may be recycled to the oxidation reactor directlyor after one or more stages of purification. The nitrogen-rich gasseparated in the separator may be compressed and passed to anotherprocess unit, such as an ammonia synthesis unit.

Alternatively the formaldehyde vent gas may be recycled directly to theprocess without treatment. In one embodiment, the formaldehyde vent gasis recycled directly to a synthesis gas generation unit as a fuel gas sothat the organic contaminants present in the vent gas may be combustedto generate energy. The formaldehyde vent gas may, for example, berecycled directly to the fuel gas stream of a primary reformer or may befed to a furnace for steam generation. In this way an ECS or vent gastreatment unit is not required, which offers considerable savings.

Alternatively, the formaldehyde vent gas may be recycled directly to acarbon dioxide removal stage so that the carbon dioxide and water vapourpresent in the vent gas may be captured. Organic contaminants such asmethanol, formaldehyde and dimethyl ether may also be captured, e.g.using a PSA unit.

Alternatively, the formaldehyde vent gas may be recycled directly to amethanol synthesis stage. Direct recycling is simpler and is preferred.With direct recycling, the by-products will be limited by equilibriumacross the methanol synthesis catalyst and so will not accumulate inthis recycle loop. The nitrogen is also recovered without the need forcatalytic combustion or intensive pressurisation.

The formaldehyde vent gas may be recycled directly to one or more ofthese alternatives.

An additional advantage of the present invention is that the absorbercolumn used in formaldehyde product recovery may be significantlysimplified. Such columns typically comprise multiple stages comprisingtrays and packing. An upper or second stage of the absorber is useful toprovide flexibility in formaldehyde product recovery but adds to thecomplexity and cost of the absorber. Because the vent gas is recycled tothe process, the present invention may permit both the omission of thevent gas treatment unit and the upper stage of the absorber, which maybe replaced with a simpler condenser.

The formaldehyde production unit may also produce an aqueous wastestream, for example a condensate recovered as a by-product of themethanol oxidation. This condensate may contain organic compounds suchas methanol, formaldehyde and dimethyl ether and therefore provides apotential source of hydrocarbon. In one embodiment, the processcondensate is recycled to a synthesis gas generation stage where it isused to generate steam for use in steam reforming. The steam may beformed in a conventional boiler and added to the hydrocarbon feed ormay, preferably, be generated in a saturator to which the aqueouseffluent and hydrocarbon are fed.

One particular embodiment of the process of the invention comprises aprocess for the production of a formaldehyde-stabilised urea productcomprising the steps of (a) generating a synthesis gas comprisinghydrogen, nitrogen, carbon monoxide, carbon dioxide and steam in asynthesis gas generation unit; (b) recovering carbon dioxide from thesynthesis gas to form a carbon dioxide-depleted synthesis gas; (c)synthesising methanol from the carbon dioxide-depleted synthesis gas ina methanol synthesis unit and recovering the methanol and a methanolsynthesis off-gas comprising nitrogen, hydrogen and residual carbonmonoxide; (d) subjecting at least a portion of the recovered methanol tooxidation with air in a formaldehyde production unit according to theinvention; (e) subjecting the methanol synthesis off-gas to methanationin a methanation reactor containing a methanation catalyst to form anammonia synthesis gas; (f) optionally synthesising ammonia from theammonia synthesis gas in an ammonia production unit and recovering theammonia; (g) reacting ammonia and at least a portion of the recoveredcarbon dioxide stream in a urea production unit to form a urea stream;and (h) stabilising the urea by mixing the urea stream and a stabiliserprepared using formaldehyde recovered from the formaldehyde productionunit.

In the methanation stage (e), residual carbon monoxide and carbondioxide in the methanol synthesis off-gas stream is converted to methanein the methanator. Any suitable arrangement for the methanator may beused. Thus the methanator may be operated adiabatically or isothermally.One or more methanators may be used. A nickel-based methanation catalystmay be used. For example, in a single methanation stage the gas from themethanol synthesis stage may be fed at an inlet temperature in the range200-400° C. to a fixed bed of pelleted nickel-containing methanationcatalyst. Such catalysts are typically pelleted compositions, comprising20-40% wt nickel. Such methanation apparatus and catalysts arecommercially available. The pressure for methanation may be in the range10-80 bar abs or higher up to 250 bar abs. Steam is formed as aby-product of methanation. The steam is desirably removed usingconventional means such as cooling and separation of condensate. Anammonia synthesis gas stream may be recovered from the methanation anddrying stage. Such methanation apparatus and catalysts are commerciallyavailable.

The methanated gas stream may be fed to an ammonia production unit asthe ammonia synthesis gas. However, the hydrogen: nitrogen molar ratioof the methanated gas stream may need to be adjusted, for example byaddition of nitrogen from a suitable source, to provide the ammoniasynthesis gas. The adjustment of the hydrogen: nitrogen molar ratio isto ensure the ammonia synthesis reaction operates efficiently. Thenitrogen may be provided from any source, for example from an airseparation unit (ASU). The adjustment may be performed by directaddition of nitrogen to the methanated gas stream. The adjusted gasmixture may then be passed to an ammonia synthesis unit as the ammoniasynthesis gas.

Ammonia may be synthesised in step (f). If ammonia is not synthesised aspart of the integrated process of the invention, then ammonia may beprovided for urea synthesis separately. When an ammonia synthesis stepis present, the ammonia synthesis gas may be compressed to the ammoniasynthesis pressure and passed to an ammonia production unit. The ammoniaproduction unit comprises an ammonia converter containing an ammoniasynthesis catalyst. The nitrogen and hydrogen react together over thecatalyst to form the ammonia product Ammonia synthesis catalysts aretypically iron based but other ammonia synthesis catalysts may be used.The reactor may operate adiabatically or may be operated isothermallyThe catalyst beds may be axial and/or radial flow and one or more bedsmay be provided within a single converter vessel. The conversion overthe catalyst is generally incomplete and so the synthesis gas istypically passed to a loop containing a partially reacted gas mixturerecovered from the ammonia converter and the resulting mixture fed tothe catalyst. The synthesis gas mixture fed to the loop may have ahydrogen: nitrogen ratio of 2.2-3.2. In the ammonia production unit, thehydrogen/nitrogen mixture may be passed over the ammonia synthesiscatalyst at high pressure, e.g. in the range 80-350 bar abs, preferably150-350 bar abs for large-scale plants, and a temperature in the range300-540° C., preferably 350-520° C.

A purge gas stream containing methane and hydrogen may be taken from theammonia synthesis loop and fed to the synthesis gas generation step orused as a fuel.

Compression of the synthesis gas is preferably effected in multiplestages, with a first and a second stage performed before the methanolsynthesis to achieve e.g. 50-100 barg, preferably 80-100 barg, and athird stage after methanation to achieve a higher pressure, e.g. 150-250barg, before the ammonia synthesis. Thus methanol synthesis may usefullybe provided between the second and third stages of compression, with themethanator downstream of methanol synthesis and upstream of the thirdstage of compression. Alternatively, the methanol synthesis may usefullybe provided upstream of the first stage of compression.

Urea may be produced in step (g) by reacting ammonia from step (f), orsupplied as a separate feed, with carbon dioxide recovered from step(d). Typically only a portion of the ammonia produced in step (f) willbe used to produce urea, which is limited by the amount of carbondioxide recovered in step (b). Excess ammonia may be recovered and usedto make nitric acid, ammonium nitrate or ammonia products for sale. Anyurea production technology may be used. For example, ammonia and carbondioxide may be combined in a first reactor at 140-200° C. and 120-220bar abs to form ammonium carbamate as follows;NH₃+CO₂

NH₂COONH₄

The ammonium carbamate is then dehydrated in a further reactor to formurea;NH₂COONH₄

NH₂CONH₂+H₂O

The high pressure favours ammonium carbamate formation and the hightemperature favours the dehydration, so the resultant mixture containsall the above components. Unreacted carbamate is therefore generallydecomposed back to ammonia and carbon dioxide, which may then berecycled to the reactor. The carbon dioxide readily dissolves in thewater from the dehydration, which if recycled supresses the equilibriaand so the system may be run with excess ammonia to minimise thisrecycle. The decomposition and subsequent recycling can be carried outin one or more successive stages at decreasing pressures to minimise theultimate concentration of ammonium carbamate dissolved in the ureasolution. An alternative process arrangement uses the fresh carbondioxide gas to strip unreacted ammonia and carbon dioxide from theammonium carbamate and urea solution at the same pressure as thereactor. Further unreacted material is recycled from lower pressurestages as ammonium carbamate solution. Such urea production apparatus iscommercially available.

Formaldehyde-stabilised urea is produced in step (h) by mixing ureaproduced in step (g) and a stabiliser prepared using formaldehyderecovered from the formaldehyde production unit in step (d). Thestabiliser may be any formaldehyde-based stabiliser; including aqueousformaldehyde and an aqueous urea-formaldehyde concentrate. Aqueousformaldehyde and urea formaldehyde concentrate may be prepared directlyin the formaldehyde production unit. Formaldehyde, either as aconcentrated solution or as a combined solution of urea and formaldehydemay be added to molten urea prior to forming into either prills orgranules. This reduces the tendency of the urea to absorb moisture andincreases the hardness of the surface of the solid particles, preventingboth caking (bonding of adjacent particles) and dusting (abrasion ofadjacent particles). This maintains the free flowing nature of theproduct; prevents loss of material through dust, and enhances thestability during long term storage. If urea is available then it ispreferable to use the urea formaldehyde solution as a stable solutionwith a higher formaldehyde concentration can be produced, whichminimises the water being added to the molten urea. Such stabilised ureaproduction apparatus is commercially available.

In FIG. 1, a natural gas stream 10, steam 16 and an air stream 12 arefed to a synthesis gas generation unit 18 comprising a primary reformer,a secondary reformer and a water-gas shift unit comprising high- andlow-temperature shift converters. The natural gas is primary reformedwith steam in externally-heated catalyst filled tubes and the primaryreformed gas subjected to secondary reforming in the secondary reformerwith air to generate a raw synthesis gas comprising nitrogen, hydrogen,carbon dioxide, carbon monoxide and steam. The steam to carbon monoxideratio of the raw synthesis gas is adjusted by steam addition ifnecessary and the gas subjected to high temperature shift and lowtemperature shift in shift converters containing high and lowtemperature shift catalysts to generate a shifted synthesis gas mixture22 in which the hydrogen and carbon dioxide contents are increased andthe steam and carbon monoxide contents decreased. Steam 20, generated bycooling the secondary and shifted gas streams, may be exported from thesynthesis gas generation unit 18. The shifted synthesis gas 22 is fed toa carbon dioxide removal unit 24 operating by means of reactiveabsorption. A carbon dioxide and water stream is recovered from theseparation unit 24 by line 26 for further use. A carbon dioxide-depletedsynthesis gas 28 comprising hydrogen, carbon monoxide and nitrogen ispassed from the carbon dioxide removal unit 24 to a methanol synthesisunit 30 comprising a methanol converter containing a bed of methanolsynthesis catalyst. If desired, upstream of the methanol synthesis unit30, steam in the shifted gas may be removed by cooling and separation ofcondensate. Methanol is synthesised in the converter and separated fromthe product gas mixture and recovered from the methanol synthesis unit30 by line 32 and passed to a formaldehyde production unit 34 comprisingan oxidation reactor containing an oxidation catalyst. Air is fed vialine 36 to the oxidation reactor where it is reacted with methanol fromline 32 to produce formaldehyde. The formaldehyde production unit is fedwith cooling water 38 and generates a steam stream 40 and a formaldehydevent gas 42. Other feed streams to the formaldehyde production unit mayinclude boiler feed water, process water and caustic (not shown). Theformaldehyde is recovered in an absorption tower which may be fed withurea, e.g. from the urea synthesis unit 64, such that either aqueousformaldehyde or a urea-formaldehyde concentrate (UFC) product stream 44may be recovered from the formaldehyde production unit 34 for furtheruse. A methanol synthesis off-gas stream 46 comprising hydrogen,nitrogen and unreacted carbon monoxide recovered from the methanolsynthesis unit 30 is passed to a methanation unit 48 comprising amethanation reactor containing a bed of methanation catalyst. Carbonoxides remaining in the off-gas 46 are converted to methane and water inthe methanation reactor. Water is recovered from the methanation unit 48by line 50. The methanated off-gas is an ammonia synthesis gascomprising essentially nitrogen and hydrogen and methane. The ammoniasynthesis gas is passed from the methanation unit 48 by line 52 to anammonia synthesis unit 54 comprising an ammonia converter containing oneor more beds of ammonia synthesis catalyst Ammonia is produced in theconverter and recovered from the ammonia synthesis unit 54 by line 56. Apurge gas stream 60 comprising methane and unreacted hydrogen andnitrogen is recovered from the ammonia synthesis unit 54 and provided tothe synthesis gas generation unit 18 as fuel and/or feed to the primaryand/or secondary reformers. A vent gas stream 62 is also recovered fromthe ammonia synthesis unit 54. A portion 58 of the ammonia is separatedfrom the product stream 56. The remaining ammonia is passed to a ureasynthesis unit 64 where it is reacted with purified carbon dioxideprovided by stream 26 to produce a urea stream and water. Water isrecovered from the urea synthesis unit 64 by line 66. The urea stream ispassed by line 68 to a stabilisation unit 70 comprising a stabilisationvessel where it is treated with aqueous formaldehyde or ureaformaldehyde concentrate provided by line 44 to form aformaldehyde-stabilised urea product. The formaldehyde-stabilised ureaproduct is recovered from the stabilisation unit 70 by line 72.

In this embodiment, the vent gas stream 42 from the formaldehydeproduction unit 34 is passed to an emission control system (ECS) 100comprising a catalytic combustor in which the organic vent gascomponents are converted to carbon dioxide and steam. The combusted gasmixture, (i.e. ECS effluent) which comprises nitrogen, carbon dioxideand steam may be suitably compressed and recycled from the emissioncontrol system 100 to the process. In one embodiment, the combusted gasmixture from the ECS unit 100 is passed by line 102 to the methanolsynthesis unit 30 where the carbon dioxide may be reacted with hydrogenin the synthesis gas to generate additional methanol. Alternatively oradditionally, the combusted gas mixture may be provided by line 106 tothe carbon dioxide removal unit 24 where the steam and carbon dioxideare removed to provide additional nitrogen in the synthesis gas.Alternatively or additionally, the combusted gas mixture may be providedvia line 104 to the urea production unit 64 where the carbon dioxide isreacted to produce additional urea.

In FIG. 2, the same synthesis gas generation, carbon dioxide removal,methanol synthesis, methanation, ammonia synthesis, urea synthesis andstabilisation units 18, 24, 30, 48, 54, 64 & 70 as set out in FIG. 1 areprovided. In this embodiment, the vent gas stream 42 from theformaldehyde production unit 34 is recycled directly, without treatmentin an ECS or other vent gas treatment units, to the process. In oneembodiment, vent gas stream is passed by line 108 to the methanolsynthesis unit 30 where the carbon dioxide is reacted with hydrogen togenerate methanol. Alternatively or in addition, the vent gas stream maybe passed by line 110 to the carbon dioxide removal unit 24 where thesteam and carbon dioxide are removed. Alternatively or in addition, thevent gas stream may be passed by line 112 to the synthesis gasgeneration unit 18 as a fuel.

What is claimed is:
 1. A process for producing formaldehyde, comprising:(a) oxidizing methanol with air in a formaldehyde production unit,thereby producing a formaldehyde-containing stream; and (b) separatingsaid formaldehyde-containing stream in a separation unit into aformaldehyde product stream and a formaldehyde vent gas stream; whereinthe formaldehyde vent gas stream, after treatment in a vent gastreatment unit, is passed to one or more stages of: (i) a synthesis gasgeneration unit, (ii) a carbon dioxide removal unit, (iii) a methanolsynthesis unit or (iv) a urea synthesis unit; wherein the vent gastreatment unit comprises an emission control system comprising acatalytic combustor to convert the vent gas stream into carbon dioxide,nitrogen, and steam.
 2. The process of claim 1, wherein the formaldehydeproduction unit comprises an oxidation reactor containing a bed ofoxidation catalyst, the process further comprising recycling a portionof the formaldehyde vent gas stream to the oxidation reactor.
 3. Theprocess of claim 1, comprising recycling the formaldehyde vent gas tothe methanol synthesis unit.
 4. The process of claim 1, comprisingpassing the formaldehyde vent gas to the carbon dioxide removal unit;wherein the carbon dioxide removal unit is a unit for removing carbondioxide from a synthesis gas generated in the synthesis gas generationunit.
 5. The process of claim 1, comprising passing the formaldehydevent gas to the urea synthesis unit.
 6. The process of claim 1,comprising passing the formaldehyde vent gas to the synthesis gasgeneration unit, wherein the formaldehyde vent gas is used in thesynthesis gas generation unit as a component of a fuel gas.
 7. Theprocess of claim 1, comprising passing the formaldehyde vent gas to thesynthesis gas generation unit, wherein the formaldehyde vent gas streamis used in the synthesis gas generation unit as a feedstock.
 8. Theprocess of claim 1, wherein the formaldehyde production unit comprisesan oxidation reactor containing a bed of oxidation catalyst, wherein theprocess is operated without recycling any of the formaldehyde vent gasstream to the oxidation reactor.
 9. A process for producingformaldehyde, comprising (a) oxidizing methanol with air in aformaldehyde production unit, thereby producing aformaldehyde-containing stream; and (b) separating saidformaldehyde-containing stream into a formaldehyde product stream and aformaldehyde vent gas stream; wherein the formaldehyde vent gas stream,optionally after treatment in a vent gas treatment unit, is passed toone or more stages of: (i) a methanol synthesis unit or (ii) a ureasynthesis unit.
 10. A process for producing formaldehyde, comprising (a)oxidizing methanol with air in a formaldehyde production unit, therebyproducing a formaldehyde-containing stream; and (b) separating saidformaldehyde-containing stream into a formaldehyde product stream and aformaldehyde vent gas stream; wherein the formaldehyde vent gas stream,without treatment in a vent gas treatment unit, is passed to one or morestages of a carbon dioxide removal unit operating by either reactiveabsorption, physical absorption, or pressure-wing adsorption.